Enhanced ionic conductivity of Cu-doped NASICON solid electrolyte for solid-state sodium batteries

Na-ion batteries using traditional liquid electrolytes face comparable challenges due to the instability and unsafety of liquid electrolytes. NASICON-type of solid electrolytes (NZSP) are promising for replacing the liquid electrolytes because of their outstanding ionic conductivity and high thermal stability. Nevertheless, the required high sintering temperature (1200 ∼ 1300 oC) with long duration in solid-state synthesis may arouse the abnormal grain growth and the volatilization of materials at the grain boundaries in NZSP. Herein, novel Cu-doped Na3+2xZr2−xCuxSi2PO12 (xCu-NZSP, x = 0.02–0.1) solid electrolytes are designed and synthesized via solid state reaction. The Cu doping can increase the concentration of Na+ carriers in the crystalline structure and reduce the sintering temperature, which results in enhanced ionic conductivity and decreased grain boundaries. The optimal 0.06Cu-NZSP possesses the highest ionic conductivity, wide electrochemical window, and stable polarization voltage due to its enhanced densification and regulated the grain size. Furthermore, the interface state between 0.06Cu-NZSP and Na metal was analyzed by the X-ray powder diffraction and X-ray photoelectron spectroscopy. The solid-state Na|0.06Cu-NZSP|Na0.67Mn0.47Ni0.33Ti0.2O2 Na-ion batteries remains a high capacity retention of 98.76% at 0.5 C after 100 cycles at 60 oC, indicating great potential of the novel 0.06Cu-NZSP in practical applications.